Method and device for adjustment of a fuel/air ratio for an internal combustion engine

ABSTRACT

In a method for adjusting the fuel/air ratio in an internal combustion engine ( 1 ) comprising a converter ( 2 ) which is associated therewith, a composition of waste gas in the waste gas wing ( 3, 8 ) of the internal combustion engine ( 1 ) is detected by means of sensors ( 4, 5 ) and output signals from at least one of the sensors ( 4, 5 ) are used for producing a control signal in order to influence the fuel/air ratio. The fuel/air ratio is switched back and forth between a lean operating state with surplus oxygen and a rich operating state with an oxygen deficit by means of a characteristic line of the control signal. The characteristic line of the control signal is adapted to a current converter state. A characteristic curve contour of the characteristic line is adjusted according to the addition and/or desorption of an oxidation agent in the converter ( 2 ). The invention also relates to a device for carrying out said method.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of copending InternationalApplication No. PCT/EP2003/014968 filed Dec. 30, 2003 which designatesthe United States, and claims priority to German application no. 102 61751.1 filed Dec. 30, 2002 and German application no. 103 10 672.3 filedMar. 12, 2003.

TECHNICAL FIELD

The invention concerns a method and device for adjustment of a fuel/airratio for an internal combustion engine.

BACKGROUND

A catalyst arranged in the exhaust system of an internal combustionengine is ordinarily used to clean the exhaust of an internal combustionengine. This converts harmful components, like hydrocarbons CH, carbonmonoxide CO and oxides of nitrogen NO_(x) essentially to nontoxic gases.It is critical to the so-called degree of conversion of the catalystthat the oxygen content of the exhaust lie within optimal values. For aso-called three-way catalyst these optimal values lie in a narrow rangearound the value corresponding to a fuel/air mixture of λ=1. In order tobe able to maintain this narrow range, it is customary to regulate thefuel/air ratio for an internal combustion engine by means of oxygensensors arranged in the exhaust system of an internal combustion engine.

A method for lambda control for internal combustion engine with adownstream catalyst is known from Unexamined Patent Application DE 40 24212 A1 in which the oxygen fractions of the exhaust of the internalcombustion engine are recorded by oxygen sensors upstream and downstreamof the catalyst. In stipulated operating ranges a control signal withcontrollable amplitude is generated by coupling in an outside signalwith controllable amplitude. With increasing catalyst aging theamplitude is reduced. The functional state of the catalyst in theexhaust system of the internal combustion engine can be determined withthe method by means of lambda regulation and the time for replacement ofan aged catalyst determined.

A method for adjustment of the fuel/air ratio for an internal combustionengine with a downstream catalyst is known from Unexamined PatentApplication DE 43 37 793 A1 in which the oxygen fractions in the exhaustof the internal combustion engine are determined by oxygen sensorsupstream and downstream of the catalyst. Both sensors influenceregulation of the fuel/air ratio. It is initially determined with anamplitude evaluation whether the catalyst has already reached a certaindegree of aging. This actual control quantity is issued by the sensorupstream of the catalyst. A switch is then made to frequency evaluationor frequency regulation in which the catalyst yields the actual controlquantity downstream of the catalyst. Such evaluations are sensitive perse, but have a relatively strong influence on the operating behavior ofthe internal combustion engine. This is avoided by only switching tofrequency evaluation when the catalyst has already reached a certainstate of aging. With increasing operating time the oxygen storagecapability of the catalyst declines. The control frequency thereforeincreases with increasing catalyst aging so that lambda regulation isadjusted to the state of aging of the catalyst. As soon as thedetermined control frequency downstream of the catalyst is higher thanthe frequency threshold, aging of the catalyst can be reliablyrecognized and the catalyst replaced.

SUMMARY

The object of the invention is to improve the method for adjusting thefuel/air ratio for an internal combustion engine with a downstreamcatalyst according to the prior art and permit greater dynamics, as wellas to devise an apparatus for execution of the method.

This object can be solved by a method for adjustment of a fuel/air ratiofor an internal combustion engine with an associated catalyst,comprising the steps of determining an exhaust composition in an exhaustsystem of the internal combustion engine by means of sensors, generatinga control signal to influence the fuel/air ratio as a function of outputsignals of at least one of the sensors, and making by means of acharacteristic curve of the control signal a switch back and forthbetween an operating state with oxygen excess and an operating statewith oxygen deficiency of the catalyst, wherein a shape of thecharacteristic curve is adjusted as a function of an oxygen and/orNO_(x) addition and/or desorption capability of the catalyst.

A course of a transition from operating state to another and/or a courseof the characteristic curve within an operating state can be adjusted asa function of oxygen and/or NO_(x) addition and/or desorption capabilityof the catalyst. The characteristic curve of the control signal can beadjusted as a function of a catalyst temperature. The characteristiccurve of the control signal can also be adjusted as a function of thedegree of aging of the catalyst. Adjustment of the characteristic curveof the control signal may occur as a function of the operatingparameters of an internal combustion engine. The characteristic curve ofthe control signal can be adjusted unsymmetrically to a stipulatedlambda value over a time range that includes at least several periods ofthe control signal. The characteristic curve of the control signal maybe a sawtooth. The subsequent operating states can be adjusted withdifferent residence time of the control signal. The subsequent operatingstates can be adjusted with different amplitude of the control signal.The characteristic curve of the control signal may be nonlinear at leastin a region. The characteristic curve of the control signal may becomeleaner or richer degressively. The characteristic of the control signalmay initially become leaner around a stipulated amount or richer andthen is degressively guided in the direction λ=1.00. The characteristiccurve of the control signal can be a rectangular curve with differentamplitudes and/or residence times in the adjusted operating states.During fuel cutoff in the overrun or idle of the internal combustionengine, the internal combustion engine may be operated more in the richoperating state than in the lean operating state. In a catalyst thecontrol signal may be adjusted so that increased incorporation of oxygenand/or NO_(x) in the catalyst occurs temporarily. In a catalyst after astipulated operating time the control signal can be adjusted so that aphase with increased lean operation follows a phase with at least twoperiods with mostly rich operation. Before reaching an operatingtemperature of the catalyst the characteristic of the control signal maydeviate from the characteristic after surpassing the operatingtemperature. In a catalyst at almost operating temperature, thecharacteristic curve of the control signal may have a sawtooth trendbefore reaching a stipulated operating temperature. The catalyst stateand/or a state of the sensor upstream and/or downstream of the catalystcan be determined from the control signal.

The object can also be achieved by a device for adjustment of a fuel/airratio for an internal combustion engine with an associated catalyst,comprising exhaust composition sensors, a control unit for generating acontrol signal to influence the fuel/air ratio as a function of outputsignals of at least one of the sensors, said control signal comprising acharacteristic curve for switching back and forth between an operatingstate with oxygen excess and an operating state with oxygen deficiencyof the catalyst, and means for adjusting a form of a characteristiccurve as a function of oxygen and/or NO_(x) addition and/or desorptionin the catalyst.

A sensor can be arranged in the exhaust system of the internalcombustion engine upstream and downstream of the catalyst. The sensorupstream of catalyst may be a broadband lambda probe with a constantcharacteristic. The sensor upstream of the catalyst can also be atwo-point lambda probe with a transfer characteristic. The sensordownstream of the catalyst may be a two-point lambda probe with atransfer characteristic. The sensor downstream of the catalyst may alsobe a broadband lambda probe with a constant characteristic. The catalystcan be a three-way catalyst. The catalyst may have a noble metal contentof less than 60 g/ft³, especially less than 40 g/ft³, preferably lessthan 30 g/ft³, optimally less than 20 g/ft³, ideally less than 10 g/ft³.The catalyst can be an NO_(x) storage catalyst. The catalyst may have anoble metal content of less than 80 g/ft³, especially less than 60g/ft³. The internal combustion engine can be a directly injectedinternal combustion engine capable of layered charging.

One advantage of the method is that the time trend of the referencevalue of lambda value of the exhaust upstream of a catalyst connectedafter the engine is automatically adjusted to the operating states ofthe catalyst in which conversion is otherwise not optimum. This leads tobetter utilization of the catalyst and to increased reliability inmaintaining emission values.

Another advantage of the invention is that because of the improveddynamics of the exhaust system in a vehicle in which the deviceaccording to the invention was implemented the driving dynamics areapproved.

In addition, the catalyst during its lifetime is brought to favorableoperating ranges and operated with high efficiency so that in comparisonwith ordinarily regulated catalysts noble metals of the catalyst can besaved and/or the catalyst itself reduced in size. This saves cost andresources.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further explained by means of drawings, in which thefigures show:

FIG. 1 shows a schematic view of a preferred device for execution of themethod according to the invention,

FIG. 2 shows a characteristic curve of a control signal according to theprior art for a three-way catalyst with new catalyst (2 a) and with agedcatalyst (2 b),

FIG. 3 shows a first preferred characteristic curve of a control signalaccording to the invention with a rectangular trend and differenttransfer height,

FIG. 4 shows a second preferred characteristic curve of a control signalaccording to the invention with a sawtooth trend,

FIG. 5 shows a third preferred characteristic curve of a control signalaccording to the invention with a degressive trend,

FIG. 6 shows a fourth preferred characteristic curve of a control signalaccording to the invention with also a degressive trend,

FIG. 7 shows a fifth preferred characteristic curve of a control signalaccording to the invention with a rectangular trend and progressiverunout,

FIG. 8 shows a sixth preferred characteristic curve of a control signalaccording to the invention with a rectangular trend and a sawtoothrunout.

DETAILED DESCRIPTION

The invention is particularly suited for catalysts in which the airfraction fed to the internal combustion engine of a fuel/air mixture isadjusted by means of a control signal that is periodically set between aminimal value and a maximal value of the air fraction and switched backand forth between a rich operating state with oxygen deficiency and alean operating state with oxygen excess.

The invention is particularly favorable for a three-way catalyst inwhich oxygen and/or NO_(x) are periodically introduced as oxidizers forthe catalyst and desorbed and in which a control signal of a lambdacontrol deviates essentially periodically around the lambda value λ=1.In the lean operating state the oxygen supply in the exhaust issufficient to oxidize its HC and CO fractions, whereas in the richoperating state NO_(x) fractions in the exhaust as oxidizers oxidize theHC and CO fractions present. A common control strategy for a three-waycatalyst proposes a lambda regulation in which a λ probe is exposed to acontrol signal with constant frequency. In the lean operating state whenλ>1 oxygen is introduced to the catalyst 2; in the rich operating statewith λ<1 this oxygen is consumed for oxidation processes.

However, it is also possible to use the invention NO_(x) storagecatalyst that can be operated at higher lambda values in a three-waycatalyst. Aged storage catalysts can also be operated at lower lambdavalues around λ=1.

FIG. 1 schematically depicts a preferred device for execution of themethod according to the invention. A catalyst 2 is connected in theexhaust 3 after the internal combustion engine 1. The internalcombustion engine 1 is supplied in the usual manner with a fuel/airmixture via means not further shown; air supply preferably occurs via anintake line 7. In the exhaust line 3 upstream of catalyst 2 a sensor 4is arranged, which detects the composition of the exhaust. The sensor 4is preferably an oxygen sensor that detects the oxygen content in theexhaust. The sensor 4 is preferably a broadband lambda probe with aconstant control characteristic. Downstream of catalyst 2 another sensoris arranged in the exhaust line 8 that can detect the composition of theexhaust purified in catalyst 2. Preferably, an oxygen sensor is alsoused here, with particular preference a two-point lambda probe with atransfer characteristic. The invention also includes devices with morethan one downstream catalyst 2.

In principle, ordinary lambda probes are suitable with sensors 4, 5,like broadband lambda probes, two-point lambda probes or NO_(x) sensorwith lambda probe function. As an alternative, a two-point lambda probecan also be used upstream of catalyst 2 and/or a broadband lambda probedownstream of catalyst 2 has sensors 4, 5. It is also conceivable todetermine the lambda value upstream of catalyst 2 from other types ofmeasured quantities, like injected amount of fuel and drawn in amount ofair.

The oxygen storage capability of catalyst 2 varies over the lifetime ofthe catalyst 2. The characteristic of a lambda probe, especially abroadband lambda probe can also vary. This can be compensated byadjusting the frequency of the control signal to the state of aging.Expediently, sensor 4 is exposed to a control signal upstream ofcatalyst 2. Sensor 5 downstream of catalyst 2 reports to sensor 4upstream as soon as breakthrough of rich exhaust or lean exhaust isobserved behind catalyst 2. As long as lean exhaust is available up tosensor 5 downstream of catalyst 2 oxygen breakthrough is recognized viathe internal combustion engine 1. A switch is made to the rich operatingstate of catalyst 2 until breakthrough of the rich component isobserved. A switch is then made back to the lean operating state and thesequence is repeated.

With increasing age the oxygen storage capability of catalyst 2diminishes, breakthroughs occur more quickly and the control frequencyrises. The lambda control is therefore adapted to the state of aging ofcatalyst. Such regulation is also referred to as natural frequencyregulation.

The sensors 4, 5 are connected to a control device 6 that receives theirsignals and sends them to evaluation. This control device 6 isexpediently a component of an ordinary engine control device used foroperation of the internal combustion engine 1. In this control device 6or via this device operating parameters of the internal combustionengine 1 or a vehicle driven by the internal combustion engine 1 areavailable. These operating parameters are preferably entered as maps ina corresponding storage medium. Such operating parameters includeexhaust temperature upstream of catalyst 2 and/or in catalyst 2, exhausttemperature downstream of the catalyst, oxygen storage capability of thecatalyst 2, exhaust flow rate, speed of the internal combustion engine1, exhaust recirculation rate, position of a camshaft disk, chargemovement flap, ignition time and/or charge pressure and the like.Information concerning the operating parameters can be linked to thesensor signals and control therefore conducted as a function of theoperating parameters. This is indicated by arrows on control device 6.Individual operating parameters or different operating parameters can beused in combination with each other.

Two characteristic curves of an ordinary control signal according to theprior art for a fresh three-way catalyst (FIG. 2 a) and an agedthree-way catalyst (2 b) are shown in FIG. 2. The frequency of thecontrol signal of fresh catalyst 2 is distinctly smaller than that ofthe aged catalyst 2. However, otherwise the characteristic curve of thecontrol signal is unchanged, since the characteristic curve shape andespecially the amplitude are retained.

Means are provided according to the invention in order to adjust acharacteristic curve of the control signal to an actual catalyst stateso that a characteristic curve shape of the characteristic curve isadjustable as a function of addition and/or desorption of an oxidizer incatalyst 2. Such a characteristic curve shape can involve a transitionfrom one operating state to another and/or a trend of the characteristiccurve within an operating state of catalyst 2. The oxidizer can beoxygen or NO_(x).

In this case the frequency of the control signal is not followed simplyas described in the prior art according to the state of aging ofcatalyst 2, but the characteristic curve shape of the characteristiccurve is varied by varying the amplitude and/or characteristic curve,especially a flank steepness, switching point and/or trend in anoperating state. This adjustment occurs within a control cycle and canvary with increasing operating time of catalyst 2. The aging behavior ofcatalyst 2 can be different in rich and lean operating states so thatconsideration of the different behavior in the two operating statespermits more efficient utilization of catalyst 2 via a correspondingadjustment of the control signal as a function of the catalyst state.

Depending on the state of aging of catalyst 2 the amplitude of thecharacteristic curve for a rich and lean operating state of the catalystcan be adjusted. This is shown in FIG. 3. An abrupt control signal withvariable amplitude is shown there. In addition, the frequency can alsobe modulated. By varying the amplitude the system can be accelerated. Ifthe sensor 5 downstream of catalyst 2 establishes a strongly depletedexhaust, it can rapidly be adjusted by stronger enrichment. Duringover-enrichment it can again be rapidly depleted. The system cantherefore reach equilibrium more quickly. The amplitudes for thetransition from the rich operating state to the lean operating state andfrom the lean operating state to the rich operating state can bedifferent.

Such a control strategy is advantageous for catalysts that have alreadybeen used for some time but are still useable over a longer time. Hereit is favorable to operate over several control periods more strongly inthe rich region, followed by a phase with mostly lean fractions and thenagain richer fractions and to repeat this. Because of this, increasedoxygen storage in the catalyst 2 can be temporarily reached up to thelimit of the regeneration capability of catalyst 2.

A trend according to FIG. 3 can also be very advantageous betweeninternal combustion engine 1 operated with thrust operated with fuelcutoff in the overrun. In this state, for example on a gradient, no fuelis temporarily fed to the internal combustion engine 1 and the internalcombustion engine 1 operates on idle. The exhaust quickly becomes lean.It is favorable here to adjust the control signal so that the internalcombustion engine is initially exposed to a fuel/air mixture that overseveral periods on a time average causes more rich fractions in theexhaust, which is recognizable by the larger amplitudes in the richoperating state. The internal combustion engine 1 is then operated inthe lean range. This permits improved dynamics of the system and betterdriving dynamics of a vehicle operated in this way.

Over a time region that includes at least several periods of the controlsignal it can therefore be advantageous to adjust the amplitudes of thecharacteristic curve of the control signal unsymmetrically to astipulated lambda value.

According to a favorable modification of the invention thecharacteristic curve of the control signal can be sawtooth. This isshown in FIG. 4. The transitions between a lean operating state to arich operating state therefore do not occur abruptly, as in the previousexample, but the transitions occur more smoothly with a finite slope ofthe flanks. This trend of the control signal is suitable for catalyst 2that has not reached or has still not reached the optimum temperaturerange, especially in the phase with average temperature between a coldstart and the sought operating temperature. It was found that thecatalyst 2 can reach its optimal temperature range for normal operationmore quickly by means of the sawtooth trend of the control signal. Theflanks of the control signal can then have different slopes in terms ofamount as well as different amplitudes.

It can prescribed that the characteristic curve of the control signal isa rectangular curve or a different characteristic curve with differentamplitude and/or residence time in the corresponding operating states sothat the percentage of rich operating states and lean operating statescan be adjusted as a function of needs to the actual state of catalyst2.

It is also possible that adjust consecutive operating states withdifferent duration depending on how dependent the addition processand/or desorption processes of the oxidizer in catalyst 2 are on theoperating parameters of the internal combustion engine and/or thelifetime of catalyst 2.

A control characteristic curve is shown in FIG. 5 that is nonlinear andhas a degressive trend. A transition from one operating state to anotheroccurs quickly with a relatively steep flank, whereupon thecharacteristic curve is slightly rising to a maximum or minimum value.The control signal can also have a progressive trend.

A control characteristic curve is shown in FIG. 6 that is nonlinear andhas a progressive trend. A transition from one operating state occursinitially quickly, but then with a relatively flat flank.

FIGS. 7 and 8 show examples of control signals, in which different curveforms are superimposed. The transitions between the operating states areabrupt but in the operating state the lean and/or fat fractions in theexhaust still increase nonlinearly or linearly. There are alsoadditional overlaps and combinations of curve forms of the controlsignal that occur in succession that can be adjusted as a function ofneed.

With increasing age the deep storage of oxygen and/or NO_(x) in catalyst2 deteriorates so that the desired catalytic processes can no longeroccur efficiently. The behavior of the catalyst 2 in lean operatingstates can be different than in rich operating states. Variation inmodulation of the characteristic curve of the control signal thereforepermits adjustment to these boundary conditions with a simultaneousincrease in efficiency of the catalytic processes. This is advantageousto maintain emission limits.

It is particularly expedient to conduct the adjustment of thecharacteristic curve of the control signal as a function of operatingparameters in internal combustion engine 1. This can occur via maps ofoperating parameters, as already described in FIG. 1. It is thereforeconsidered that the behavior of catalyst 2 is strongly influenced by theoperating parameters of the internal combustion engine 1. The rate ofconversion changes sharply with exhaust temperature. The catalyst 2 isexposed to pollutants with a high exhaust flow rate so that in theextreme case the purification efficiency of catalyst 2 can decline. Ifexhaust recirculation is varied, the richness of the fuel/air mixturechanges. If the amount of recycled exhaust rises, the NO_(x) content inthe exhaust drops. The ignition point influences the system in similarfashion to exhaust recirculation. By influencing the combustion trendthe pollutant and oxygen concentration change at the same lambda value.With low crude emissions or a shift to more readily convertiblepollutants (for example more CO, less CH₄) the requirements on accuracyof lambda control diminish. Such influences of the operating parameterscan be considered by corresponding adjustment of the characteristiccurve of the control signal so that maintenance of the emission standardis ensured over broad operating ranges of the internal combustion engine1.

In addition to ensuring the emission standard, in an advantageousembodiment of the invention the catalyst state and/or state of thesensor 4, 5, especially sensor 5 downstream from catalyst 2 can bedetermined from a change in the characteristic curve of the controlsignal.

In addition to increased reliability in maintain in emission values, theinvention also permits a reduction in noble metal content of catalyst 2.The catalyst 2 is mostly operated in regions in which conversion isimproved. Because of this the catalyst volume can be correspondinglyreduced and/or the noble metal compound of the catalyst can be reducedin order to achieve the same efficiency as in an ordinary control. It ispossible to reduce the noble metal content and/or the catalyst volumeduring use of the method according to the invention without surpassingthe pollutant emissions forming without use of the method by at least10%, especially by at least 20%. In particular, the catalyst 2 in thecase of an NO_(x) storage catalyst has a noble metal content of less 80g/ft³, especially less than 60 g/ft³. In a three-way catalyst a noblemetal content of less than 60 g/ft³, especially less than 40 g/ft³,preferably less than 30 g/ft³, optimally less than 20 g/ft³, ideallyless than 10 g/ft³ is provided.

1. A method for adjustment of a fuel/air ratio for an internalcombustion engine with an associated catalyst, comprising the steps of:determining an exhaust composition in an exhaust system of the internalcombustion engine by means of sensors, generating a control signal toinfluence the fuel/air ratio as a function of output signals of at leastone of the sensors, and making by means of a characteristic curve of thecontrol signal a switch back and forth between an operating state withoxygen excess and an operating state with oxygen deficiency of thecatalyst, wherein a shape of the characteristic curve is adjusted as afunction of an oxygen and/or NO_(x) addition and/or desorptioncapability of the catalyst.
 2. A method according to claim 1, wherein acourse of a transition from operating state to another and/or a courseof the characteristic curve within an operating state is adjusted as afunction of oxygen and/or NO_(x) addition and/or desorption capabilityof the catalyst.
 3. A method according to claim 1, wherein thecharacteristic curve of the control signal is adjusted as a function ofa catalyst temperature.
 4. A method according to claim 1, wherein thecharacteristic curve of the control signal is adjusted as a function ofthe degree of aging of the catalyst.
 5. A method according to claim 1,wherein adjustment of the characteristic curve of the control signaloccurs as a function of the operating parameters of an internalcombustion engine.
 6. A method according to claim 1, wherein thecharacteristic curve of the control signal is adjusted unsymmetricallyto a stipulated lambda value over a time range that includes at leastseveral periods of the control signal.
 7. A method according to claim 1,wherein the characteristic curve of the control signal is a sawtooth. 8.A method according to claim 1, wherein the subsequent operating statesare adjusted with different residence time of the control signal.
 9. Amethod according to claim 1, wherein the subsequent operating states areadjusted with different amplitude of the control signal.
 10. A methodaccording to claim 1, wherein the characteristic curve of the controlsignal is nonlinear at least in a region.
 11. A method according toclaim 10, wherein the characteristic curve of the control signal becomesleaner or richer degressively.
 12. A method according to claim 11,wherein the characteristic of the control signal initially becomesleaner around a stipulated amount or richer and then is degressivelyguided in the direction λ=1.00.
 13. A method according to claim 1,wherein the characteristic curve of the control signal is a rectangularcurve with different amplitudes and/or residence times in the adjustedoperating states.
 14. A method according to claim 1, wherein during fuelcutoff in the overrun or idle of the internal combustion engine, theinternal combustion engine is operated more in the rich operating statethan in the lean operating state.
 15. A method according to claim 1,wherein in a catalyst the control signal is adjusted so that increasedincorporation of oxygen and/or NO_(x) in the catalyst occurstemporarily.
 16. A method according to claim 1, wherein in a catalystafter a stipulated operating time the control signal is adjusted so thata phase with increased lean operation follows a phase with at least twoperiods with mostly rich operation.
 17. A method according to claim 1,wherein before reaching an operating temperature of the catalyst thecharacteristic of the control signal deviates from the characteristicafter surpassing the operating temperature.
 18. A method according toclaim 1, wherein in a catalyst at almost operating temperature, thecharacteristic curve of the control signal has a sawtooth trend beforereaching a stipulated operating temperature.
 19. A method according toclaim 1, wherein the catalyst state and/or a state of the sensorupstream and/or downstream of the catalyst is determined from thecontrol signal.
 20. A device for adjustment of a fuel/air ratio for aninternal combustion engine with an associated catalyst, comprising:exhaust composition sensors, a control unit for generating a controlsignal to influence the fuel/air ratio as a function of output signalsof at least one of the sensors, said control signal comprising acharacteristic curve for switching back and forth between an operatingstate with oxygen excess and an operating state with oxygen deficiencyof the catalyst, and means for adjusting a form of a characteristiccurve as a function of oxygen and/or NO_(x) addition and/or desorptionin the catalyst.
 21. A device according to claim 20, wherein a sensor isarranged in the exhaust system of the internal combustion engineupstream and downstream of the catalyst.
 22. A device according to claim21, wherein the sensor upstream of catalyst is a broadband lambda probewith a constant characteristic.
 23. A device according to claim 22,wherein the sensor upstream of the catalyst is a two-point lambda probewith a transfer characteristic.
 24. A device according to claim 21,wherein the sensor downstream of the catalyst is a two-point lambdaprobe with a transfer characteristic.
 25. A device according to claim21, wherein the sensor downstream of the catalyst is a broadband lambdaprobe with a constant characteristic.
 26. A device according to claim20, wherein the catalyst is a three-way catalyst.
 27. A device accordingto claim 26, wherein the catalyst has a noble metal content of less than60 g/ft³, especially less than 40 g/ft³, preferably less than 30 g/ft³,optimally less than 20 g/ft³, ideally less than 10 g/ft³.
 28. A deviceaccording to claim 20, wherein the catalyst is an NO_(x) storagecatalyst.
 29. A device according to claim 28, wherein the catalyst has anoble metal content of less than 80 g/ft³, especially less than 60g/ft³.
 30. A device according to claim 20, wherein the internalcombustion engine is a directly injected internal combustion enginecapable of layered charging.